In general, transparent films comprising polyethylene terephthalate (PET), acryl, polycarbonate (PC), cellulose triacetate (TAC), polyolefin and the like are used as base materials of optical functional films used for components of displays such as Liquid Crystal Display (LCD) and Plasma Display Panel (PDP). When these base films are used for various optical functional films, a functional layer suitable for each purpose is laminated on each of the base films. For example, with regard to Liquid Crystal Display (LCD), there can be listed functional layers such as a protection film (hard coat layer) for preventing scratches on the surface, an antireflection layer (AR layer) for preventing reflection of outside light, a prism layer used for collection and diffusion of light, a light diffusion layer for improving brightness. Among these base materials, the biaxially oriented polyester films are widely used as a base material for various optical functional films due to transparency, dimensional stability and chemical resistance thereof.
In general, in a case of a biaxially oriented thermoplastic film such as a biaxially oriented polyester film and a biaxially oriented polyamide film, the film surface has a highly crystalline orientation, so that there is a defect that it lacks adhesion to various paints, adhesives, inks and the like. For this reason, there have been proposed various methods for providing an easy adhesion to a biaxially oriented thermoplastic resin film surface.
Further, a film devoid of a polar group, such as polyolefin film, is extremely poor in adhesion to various paints, adhesives, inks and the like, so that there has been proposed various methods for providing easy adhesion to a film surface after performing a preliminary physical or chemical treatment such as corona discharge treatment and flame treatment.
For example, a method of providing easy adhesion to a base film by providing a covering layer containing various resins such as a polyester, an acrylic, a polyurethane and acrylic graft polyester resin as main constituting components on a surface of a thermoplastic resin film which is a base material by using one of coating methods is generally known. Among the coating methods, industrially and widely performed methods include a method wherein corona discharge treatment is carried out on a thermoplastic resin film prior to completion of crystalline orientation directly or when necessary and then an aqueous coating liquid containing a dispersion in which the resin solution or the resin is dispersed with a dispersion medium is coated on a base material film, and after drying, the film is stretched at least in a uniaxial direction followed by being subjected to heat treatment to complete crystalline orientation of the thermoplastic resin film (so called inline coating method), and a method wherein a water-type or solvent-type coating liquid is coated on a thermoplastic resin film after production of the film followed by drying (so called offline coating method).
Year after year, displays such as LCD and PDP are getting larger and costs thereof are getting lower. In a step for producing an optical functional film or a optical functional sheet used as the material thereof, production speed has been increased. Accompanying such speeding up of production processes, stress resulting from hardening and contraction is more apt to occur at an interface between functional layers such as hard coat layer, a diffusion layer, and a prism layer and a base film. Therefore, there has arisen a problem that when cutting an optical functional film or an optical functional sheet in a specific size in order to produce a display, an end thereof is particularly easy to peel off if adhesion at the interface is insufficient. With respect to this tendency, as a size of a film wound into a roll and a production speed in production processes increase, effects of delamination (peeling) of an interface with an impact of cutting become conspicuous, so that a conventional level of adhesion is no longer sufficient.
Further, a processing agent to be used for forming the functional layer such as the prism layer and diffusion layer are often directly coated on a base material film without being diluted with an organic solvent from a viewpoint of reducing burdens on environment. Therefore, since an improvement effect of the organic solvent on wettability of the covering layer sometimes cannot be obtained sufficiently, a higher adhesion is required. On the other hand, in a usage which places importance on smoothness such as hard coating, in order to reduce viscosity of the processing agent to obtain a good leveling effect, the processing agent is often diluted with an organic solvent. In such a case, the covering layer of the laminated thermoplastic resin film is required to have an appropriate solvent resistance.
In order to enhance adhesion between a functional layer and a base film, a method using a resin having low glass transition temperature for a resin constituting a covering layer is generally employed. However, when using a resin having a low glass transition temperature, the blocking resistance tends to be lowered when winding the film sequentially into a roll and winding off the film from the film roll.
In recent years, processing machines for laminating functional layers such as a hard coat layer and a diffusion layer on a base film are getting larger in order to reduce costs, and diameters of easy adhesion film rolls used as base films are getting larger. Accompanying this, when winding the film by high tension in order to prevent roll misalignment, blocking will be more apt to occur particularly at an wound core of the roll since it is pressure-bonded by high pressure.
In order to improve blocking resistance, a method providing concavity and convexity on a film surface to reduce the contact area is generally employed. In order to provide concavity and convexity on the film surface, a method increasing a content of inorganic particles or organic particles to be contained in the covering layer or the base material film, or using particles having a large particle size is commonly used. However, a refractive index of generally available particles is different from a refractive index of a resin used for the covering layer, and with a step for stretching, a void is formed around the particles, so that lowering of light transmission of the film, increase in haze and the like occur in these methods. In particular, transparency which is required in a base material film of an optical functional film or an optical functional sheet is lowered. Namely, the new problems accompanying an increase in speed of the processes and in the diameter of the film roll made it extremely difficult to improve, by a conventional method, adhesion to and blocking resistance against the functional layers while maintaining transparency.
On the other hand, a use of information terminals outdoors, such as cellular telephones, PDAs and mobile type computers, is becoming more frequent. Further, materials used in a car which becomes high in temperature in summer, such as a touch panel used for car navigation and the like, are increasing. Therefore, a film which is small in fluctuation of qualities in such a severe environment at high temperature and humid, i.e., a film which is excellent in humidity- and heat-resistant adhesion is demanded for such a usage.
In particular, a biaxially oriented polyester film is known for having poor adhesion to a coating agent comprising an acrylic resin as a main component used for a prism lens, hard coating and the like. For this reason, various polyester films formed with a covering layer comprising a polyurethane resin and the like on a surface thereof have been proposed (e.g. refer to Patent Document 1: Japanese unexamined patent publication No. H6-340049). However, in a film formed with a covering layer comprising a polyurethane resin, even though adhesive strength to a functional layer such as hard coat layer is improved, adhesive strength to a polyester film which is a base material is not sufficient, resulting in a problem of insufficient adhesion at an interface between the covering layer and the functional layer. Additionally, the resin constituting the covering layer is low in degree of cross-linking and poor in humidity- and heat-resistant adhesion, so the demand of the market sometimes could not be satisfied sufficiently in a usage which strongly requires humidity- and heat-resistant adhesion.
Further, there have been proposed methods of improving adhesion between a base polyester film and a functional layer such as an ink by inline coating method wherein a resin composition layer comprising a polyester resin and a polyurethane resin as a main constituting component is provided on the base polyester film comprising biaxially oriented polyethylene terephthalate (e.g., refer to Patent Document 2: Examined patent publication No. 64-6025). More specifically, after a water dispersible coating liquid containing a copolymerized polyester resin and a polyurethane resin (=20/80; % by mass) was coated on a polyester film uniaxially-stretched in a longitudinal direction, the polyester film was introduced to a tenter and was subjected to drying, transversal stretching, and heat fixation at 220° C. to obtain an easy adhesion biaxially oriented polyester film.
However, even though adhesion was improved in the method described in Patent Document 2, the method could not simultaneously satisfy adhesion to a hard coat layer, a diffusion layer and a functional layer, blocking resistance, and transparency of base film demanded in recent years as a base film used for an optical functional film or an optical functional sheet. Additionally, due to a low degree of cross-linking of a resin constituting the covering layer, and poor humidity- and heat-resistant adhesion, it sometimes could not satisfy the demand of the market sufficiently in a usage wherein humidity- and heat-resistant adhesion was strongly required.
The present applicant proposed a laminated polyester film provided with a resin composition layer on a base film comprising biaxially oriented polyethylene terephthalate containing a polyester resin, a polyurethane resin, and inorganic particles having an appropriate particle size added thereto and which can sufficiently satisfy adhesion at a level required by the market and has fewer optical defects while maintaining transparency that is an extremely important characteristic as an optical base film (e.g. refer to Patent Document 3: Japanese unexamined patent publication No. 2000-323271, Patent Document 4: Japanese unexamined patent publication No. 2000-246855). Specifically, it discloses an easy-adhesion biaxially oriented polyester film obtained by coating a water dispersible coating liquid containing a copolymerized polyester resin, a polyurethane resin (=20/80; % by mass), two kinds of silica particles having different average particle sizes and an anionic surfactant on a polyester film uniaxially-stretched in a longitudinal direction, followed by introducing the polyester film into a tenter for subjecting it to drying, transversal stretching and heat fixation at 240° C.
The easy adhesion biaxially oriented polyester films obtained in Patent Documents 3 and 4 had excellent adhesion, blocking resistance and transparency, and optical defects thereof such as foreign matter and scratches were considerably improved, satisfying properties heretofore required. However, as described above, with reduction of costs and increasing sizes of displays in recent years, the request levels of adhesion between a base film and a functional layer such as a hard coat layer, a diffusion layer, and a prism layer as well as of blocking resistance required as a base film for an optical functional film or an optical functional sheet tend to be getting higher year by year; therefore, they can no longer satisfy the qualities demanded by the market today. Additionally, due to a low degree of cross-linking of a resin constituting the covering layer and poor humidity- and heat-resistant adhesion, the demand of the market sometimes could not be satisfied sufficiently in a usage wherein humidity- and heat-resistant adhesion is strongly required.
Additionally, the present applicant proposed an invention regarding an easy adhesion film roll with reduced fluctuation of an amount of coating in order to improve uniformity of adhesion (e.g. refer to Patent Document 5: Japanese unexamined patent publication No. 2004-10669). In Examples of Patent Document 5, there is described an easy adhesion biaxially oriented polyester film obtained by coating a water dispersible coating liquid containing a polyester resin, a polyurethane resin (=50/50; % by mass), silica particles having an average particle size of 1.4 μm and a fluorinated surfactant on a polyester film uniaxially-stretched in a longitudinal direction, followed by subjecting the polyester film to drying at 120° C. in a drying furnace, transversal stretching and subsequently treating by heat fixation at 220° C. The resultant film roll, had an excellent adhesion uniformly in an entire film roll, sufficiently satisfying the level required by the market. However, as described above, it can no longer satisfy the blocking resistance required in recent years sufficiently. Further, due to a low degree of cross-linking of a resin constituting covering layer and poor humidity- and heat-resistant adhesion, the demand of the market sometimes could not be satisfied sufficiently in a usage wherein humidity- and heat-resistant adhesion is strongly required.
Moreover, there is disclosed a biaxially stretched film obtained by coating a coating liquid containing 100 parts by mass of a polyester comprising telephthalic acid/isophthalic acid/trimellitic acid/sebacic acid as acid components and ethylene glycol/neopentyl glycol/1,4-butanediol as glycol components, 15 parts by mass of a methylol-type melamine cross-linker, and 0.7 part by mass of particles on a polyethylene terephthalate film uniaxially-stretched in a longitudinal direction, and further by subjecting it to transversal stretching, heat fixation, and relaxation treatment (e.g., refer to Patent Document 6: Japanese unexamined patent publication No. 2004-299101). However, even though this method provided an improved humidity- and heat-resistant adhesion, it was poor in initial adhesive strength to a functional layer such as a hard coat layer and could not satisfy sufficiently the quality required for an optical film in recent years.
Namely, conventional art is becoming no longer satisfactory enough in terms of adhesion for resisting high speed cutting which is demanded in recent years and in terms of blocking resistance for responding to the increasing size of a film roll while maintaining high transparency. Further, it was not satisfactory enough with respect to humidity- and heat-resistant adhesion which means less decrease in adhesion even in a harsh environment of high temperature and humid.